1. Description of the Prior Art
Recent developments in switching elements that can be used at high frequencies have resulted in an increase in the switching frequencies of DC--DC converters, and this in turn has led to expectations of reductions in the size of DC--DC converters because it should now be possible to construct smaller versions of the transformers, choke coils, and smoothing capacitors that take up so much room conventionally.
However, the switching losses that occur as current and voltage are superimposed whenever switching elements turn on and off increase as frequencies increase. This means that, regardless of how small these components and other circuit elements become, at present the heat dissipation countermeasures necessary for coping with the heat generated by switching losses ensure that sizes cannot be reduced further.
As the frequencies of DC--DC converters have increased, it has become common to use insulated-gate field-effect transistors. However, insulated-gate field-effect transistors have the problem of parasitic capacitances; if such a transistor is switched on or off while voltage is applied thereto, the parasitic capacitance causes a short circuit, which generates noise. Therefore, countermeasures against this noise are also necessary.
A circuit diagram of a conventional forward DC--DC converter with one switching transistor is shown in FIG. 4, and the timings of voltage and current waveforms in this DC--DC converter are shown in FIG. 5.
In this DC--DC converter, a DC source E.sub.s, a primary winding L.sub.1 of a transformer T.sub.1, and an insulated-gate field-effect transistor Q.sub.1 that acts as a switching element form a series circuit, and a rectifying and smoothing circuit formed of a rectifier diode D.sub.1, a flywheel diode D.sub.2, a choke coil L.sub.3, and a smoothing capacitor C.sub.1 is connected to a secondary winding L.sub.2 of the transformer T.sub.1.
The transistor Q.sub.1 receives a gate voltage from a control circuit that is not shown in the figure.
While the transistor Q.sub.1 of the above DC--DC converter is on, a current flows through the primary winding L.sub.1 on the input side of the transformer T.sub.1, and a DC output is obtained at output terminals 1 and 1' from a voltage induced in the secondary winding L.sub.2 on the output side, using the rectifying and smoothing circuit.
FIG. 5 shows the waveforms of the gate voltage V.sub.G1 of the transistor Q.sub.1, the drain-source voltage V.sub.Q1 of the transistor Q.sub.1, and the current I.sub.Q1 flowing through the transistor Q.sub.1 via the primary winding L.sub.1, expressed against the same horizontal time axis. As can be seen from the figure, the drain-source voltage V.sub.Q1 and the current I.sub.Ql are superimposed during a period between a time t.sub.1 at which the transistor Q.sub.1 turns on and a subsequent time t.sub.2, and during a period between a time t.sub.3 at which the transistor Q.sub.1 turns off and a subsequent time t.sub.4. This superimposition causes power losses.